The invention relates to an integrated semiconductor circuit comprising a semiconductor body with a layer-shaped region of a first conductivity type adjoining a surface and limited at the side remote from the surface by a region, called substrate hereinafter, of the second conductivity type opposite to the first conductivity type, one or several insulated islands being formed in the layer-shaped region with one or several circuit elements comprising at least a diode with a first main electrode region in the form of a surface zone of the second conductivity type provided in the layer-shaped region and with a second main electrode region of the first conductivity type comprising a portion of the layer-shaped region adjoining the first main electrode region and connected to a source region of a junction field effect transistor (JFET) whose channel is formed by a portion of the layer-shaped region situated adjacent the diode. Such a device is known, for example, from U.S. Pat. No. 4,494,134.
In most versions, the layer-shaped region is formed by an n-type surface layer of silicon which was deposited epitaxially on a p-type silicon substrate. The islands are usually separated from one another by interposed p-type zones which extend from the surface transversely through the thickness of the epitaxial layer (referred to as epi layer hereinafter for short). In the case of high-voltage circuits, for which the invention is of particular importance, and where the thickness of the epi layer may be very considerable, for example 20 .mu.m, these zones may be composed from a surface zone and a buried zone. Obviously, the island insulation may be formed in a different manner completely or partly, for example, by means of dielectric insulation. The diode may be used purely as a rectifier, for example, in the case of a so-called bootstrap in which a junction point in the circuit is charged to a certain comparatively low voltage through the diode and subsequently is brought to a high voltage level, which may be higher than the supply voltage, by means of a capacitive coupling. The first main electrode region formed by a p-type surface zone then forms the anode while the second main electrode region formed by the n-type epi layer forms the cathode. In alternative versions, the diode may form part of a circuit element, for example, the base-collector junction of a bipolar transistor in which an n-type zone forming the transistor emitter is provided in the zone which forms the said first main electrode region and which is of the p-type when the epi layer is of the n-type.
The U.S. Pat. No. 4,494,134 cited above describes a diode in which a highly doped buried n-type layer is provided between the n-type epi layer and the p-type substrate, below the anode. It is an object of this buried layer to suppress the parasitic pnp action to the substrate when the diode is forward biased. Simultaneously, the breakdown voltage of the pn junction may be strongly reduced as a result of the buried layer, so that the diode breaks down in the reverse direction at a voltage which is too low for many applications. To avoid this problem, a p-type surface zone is provided between the anode and the cathode, forming the gate of a junction field effect transistor or JFET whose source is formed by the n-type region around the anode and whose drain is formed by the cathode in the form of an n-type zone surrounded by the gate electrode. The gate is in its turn surrounded by the anode which is provided along the edge of the island. When a voltage is supplied through the drain, the source will follow until the channel of the JFET is pinched off. A further increase of the voltage at the drain will not or hardly not lead to an increase in the voltage at the source. In this manner the voltage across the diode in the reverse direction can be limited to a comparatively low value, i.e. the pinch-off voltage of the JFET. A disadvantage of the known device is that it occupies disproportionately much space, especially when the anode itself is surrounded in the epi layer by a cup-shaped n.sup.+ zone for preventing lateral injection of holes into the island insulation. In addition, a diode of only a small surface area is often required, which in the concentric configuration described here, with the anode situated at the outside, is difficult to realize.